1. Field of the Invention
The present invention relates to a scanning optical apparatus that is used in image forming apparatuses such as an electrophotographic copying machine and a laser printer.
2. Description of the Related Art
In a scanning optical apparatus that is used in an image forming apparatus such as an electrophotographic copying machine and a laser printer, generally, an optical beam emitted from a light source is deflected by a deflector and is condensed by an imaging optical system such as an f-theta lens to form a beam spot on a photosensitive drum. The scanning optical apparatus scans the photosensitive drum surface with the beam spot.
A laser beam deflected and scanned within an optical box forms an image using an optical lens, and the image is reflected by reflecting mirrors toward the photo sensitive drum. In the process, if a relative position of the optical lens is deviated by a vibration or deformation of the optical box due to temperature change, an illumination position of the laser beam toward the drum surface is deviated. A positional deviation of images is roughly classified into a parallel deviation, an oblique deviation, and a quadratic curve deviation.
Therefore, the optical box is required for example, to have lowest resonance frequency which is more than a frequency of a drive source, to enhance resistance to a vibration, not to have a natural frequency near a frequency of a drive source, or not to generate deformation in directions other than a linear expansion direction if environmental temperatures vary.
At the same time, when a small deformation is generated in a body frame, it is necessary to ensure adequate static stiffness which does not generate in the optical box deformation greater than the deformation in the body frame.
However, within the optical box, a deflection motor is disposed, and many optical components such as lenses and reflection mirrors are disposed around the deflection motor. Further, optical paths in each optical system need to be ensured in the optical box, so that areas for forming rib structure for reinforcement are limited.
To solve the problem, for example, Japanese Patent Application Laid-Open No. 2006-234977 discusses providing a reinforcing member for connecting ribs provided around a deflection motor on a center side of an optical box as a method to increase strength of a scanning optical apparatus other than providing ribs within the optical box.
The structure discussed in Japanese Patent Application Laid-Open No. 2006-234977 is effective to cope with a self-excited vibration due to rotation of the deflection motor. However, the structure is not sufficient to secure a vibration-proof performance against an external vibration and to reduce torsion of a frame of the optical box that causes positional deviation of an optical element.
To solve the problem, as a reinforcing member, a covering member can be provided that covers an entire opening surface facing a base plate of a scanning optical apparatus. However, such a covering member has the following problem. That is, an optical axis needs to be adjusted to a light flux emitted from the optical box after the assembly of the optical box is completed. However, adjustment of the optical axis is not physically possible in the state that the covering member is mounted. It is conceivable that the covering member is removed to perform the adjustment and then mounted on the apparatus after completion of the adjustment. However, the high-stiffness covering member mounted over the entire optical box upper surface will cause deformation of the optical box due to fitting of the optical box to the shape of the covering member if the covering member has even a small problem with accuracy. Accordingly, the adjusted optical axis can be deviated.